1. Field of the Invention
The invention relates to aircraft gas turbine engines with single stage fans or counter-rotatable fan stages and, particularly, for such engines having boosters or boosters between the counter-rotatable fan stages.
2. Description of Related Art
A gas turbine engine of the turbofan type generally includes a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine. The core engine includes a high pressure compressor, a combustor, and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high pressure compressor, turbine, and shaft essentially form the high pressure rotor. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aft and passes through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. Some fan jet engines have been designed with counter-rotatable turbines that power counter-rotatable fans and boosters or low pressure compressors. U.S. Pat. Nos. 4,790,133, 4,860,537, 5,307,622 and 6,732,502 disclose counter-rotatable low pressure turbines (LPT) that power counter-rotatable fans and booster or low pressure compressors. Most of the thrust produced is generated by the fan. There are also various designs for counter-rotatable fan engines that use gearboxes to effect counter-rotation of the fans and boosters.
Boosters typically have rotatable booster blades extending radially outwardly from a radially inner rotatable drum or rotor. The rotatable blades are interdigitated with non-rotatable booster vanes extend radially inwardly from a non-rotatable outer shell, drum, or duct. Among the drawbacks of this design is that the rotatable booster blades are more susceptible to rubs during engine accelerations. Thus, an alternative more robust engine design or configuration is desirable in order to minimize these rubs.